A conventional technique for operating an occupant protection device in a vehicle is configured in the following manner, according to the disclosure of a patent document 1 (i.e., JP-A-2009-96394), for example. That is, in such technique, a vehicle body crash amount is estimated in a vehicle collision based on the collision type, and then a calculation of kinetic energy of an occupant in the vehicle is performed based on a weight of the occupant and a vehicle speed at the time of collision. Also, a calculation of an occupant retain force for retaining the occupant is performed based on a final vehicle body crash amount, the kinetic energy of the occupant, a current amount of the crash of the vehicle body and a distance between the occupant and a vehicle interior, for the operation of an occupant protection unit of the occupant protection device. Such an occupant protection device retains the occupant according to a collision state of the vehicle, i.e., a collision direction, a collision part, a collision speed and the like, thereby enabling and providing an appropriate occupant protection for the occupant of the vehicle.
An offset collision is one of many forms of a head-on collision. Many vehicle occupant protection devices have been proposed for protecting the occupant from, such a collision. The offset collision is a collision in which a portion of a vehicle front face collides with another vehicle or an obstacle. In one aspect, the offset collision has a slightly longer collision time than a full-width collision, causing an impact of the collision mitigated in the course of crash of the vehicle body. However, the impact of the offset collision is concentrated to a small portion of the vehicle body, thereby making the offset collision more dangerous than other forms of collisions because the cabin space for the occupant in the vehicle may collapse in the offset collision. For coping with the dangerousness of the offset collision, many vehicle body structures have been proposed, in which the impact of the offset collision of the vehicle is diverted from a smaller portion to other parts for keeping the vehicle compartment (i.e., the cabin space of the occupant) intact.
However, the hazards to the vehicle occupant in the offset collision are not limited to the above-described crash and resultant decrease of cabin space. That is, the vehicle in the offset collision has an angular moment, which causes a spin of the vehicle body. Particularly, when the vehicle is in a small overlap collision (i.e., Small Overlap Impact, or SOI) in which an amount of overlap between the own vehicle and an obstacle is small, a rotation speed and a rotation angle of the vehicle body are high. Therefore, to prevent such a hazard, an occupant protection device such as a side airbag, a curtain airbag and the like have been proposed, for the improved protection of the occupant in such a collision.
Normally, the side airbag is operated, i.e., deployed, when a side acceleration or a front-rear acceleration caused by the impact in the collision is equal to or greater than a preset threshold. However, the offset collision makes it difficult to accurately predict the rotation direction and the rotation speed of the vehicle, based only on the impact that is picked up/input by the impact detection sensors on various parts of the vehicle at an early stage of the collision. In view of such a situation, the interior members and surfaces of the vehicle are changed to a non-damaging, soft material and structure for protecting the body of the occupant and/or reducing the injury of the occupant. Further, for preventing the occupant from being thrown out of the vehicle at the time of the vehicle collision, the use of the seat belt has been advised for the vehicle occupants including rear seat passengers (i.e., mandated in Japan).